Performance Optimization in PWAs: Core Web Vitals Quiz Quiz

Enhance your Progressive Web App expertise with this quiz focused on Core Web Vitals optimization. Evaluate your understanding of key performance metrics, optimization techniques, and best practices essential for achieving top scores in PWA performance.

1. Largest Contentful Paint Optimization

   Which change most directly improves the Largest Contentful Paint (LCP) score in a PWA that displays a large image banner at the top?

   1. Compressing and serving the banner image in modern formats
   2. Using a custom font for headings
   3. Preloading all images on page load
   4. Adding animations to buttons

   Explanation: Compressing and delivering images in modern formats reduces load times, directly benefitting the LCP, which measures how quickly the main content is visible. Using a custom font for headings mostly affects text rendering, not LCP. Adding animations may increase rendering complexity without improving loading time. Preloading all images can waste bandwidth and delay critical resources, harming initial load performance.

2. First Input Delay Scenario

   If users notice a delay after clicking a button before any response occurs in your PWA, which Core Web Vitals metric is primarily impacted?

   1. Total Blocking Time (TBT)
   2. Cumulative Layout Shift (CLS)
   3. First Input Delay (FID)
   4. Time to Interactive (TTI)

   Explanation: First Input Delay (FID) measures the time between a user's first interaction and the browser's response, making it the most relevant metric when users experience input lag. Cumulative Layout Shift (CLS) relates to visual layout stability, not responsiveness. Time to Interactive (TTI) indicates when a page becomes fully interactive, but may not reflect individual delays. Total Blocking Time (TBT) measures the amount of time main thread work blocks response, but FID specifically captures input response delay for the user.

3. Avoiding Layout Shifts

   When building a PWA, which action most effectively prevents unexpected layout shifts that negatively affect the Cumulative Layout Shift (CLS) metric?

   1. Deferring all scripts until after page load
   2. Using only vector images
   3. Always setting explicit width and height for images
   4. Disabling custom fonts

   Explanation: Explicitly defining image dimensions reserves space during loading, preventing sudden shifts that impact CLS. Using only vector images may improve scalability but does not guarantee layout stability. While deferring scripts can improve performance, it does not directly resolve layout shifts. Disabling custom fonts may improve text rendering speed but does not address shifting caused by undisclosed image sizes.

4. Improving PWA Interactivity

   In a PWA with complex JavaScript, what is a recommended method to enhance responsiveness and reduce input delay for Core Web Vitals optimization?

   1. Breaking up long tasks into smaller asynchronous chunks
   2. Combining all scripts into a single large file
   3. Switching all images to SVG format
   4. Increasing the size of service worker caches

   Explanation: Splitting long JavaScript tasks allows the browser to respond to user interactions more quickly, reducing input delay and aiding Core Web Vitals scores. Combining scripts into one large file can increase parsing time and block interactivity. Larger service worker caches improve offline abilities, not responsiveness. Switching images to SVG affects image quality and scalability, but does not impact JavaScript execution or input delay.

5. Critical CSS Delivery

   Why is delivering critical CSS inline recommended for boosting Core Web Vitals, particularly in Progressive Web Apps?

   1. It ensures essential styles are applied immediately, improving load speed and visual stability
   2. It creates larger HTML files, which always improves SEO rankings
   3. It replaces the need for JavaScript in the PWA
   4. It prevents browsers from rendering any content until all resources are loaded

   Explanation: Inlining critical CSS ensures above-the-fold content is styled as soon as possible, enhancing perceived load performance and stability, which is vital for Core Web Vitals. Creating larger HTML files is not inherently beneficial and can have downsides. CSS delivery does not replace JavaScript, and inlining doesn't block rendering of other content; rather, it speeds up the initial rendering process.